1. Field of the Invention
This invention relates to a tracking error detection apparatus for a recording and/or reproducing apparatus, in which a light beam is irradiated by an optical pickup on an optical disk to read out data signals from a track on the optical disk.
2. Prior Art
In an optical disk recording and/or reproducing apparatus in which a light beam is irradiated on an optical disk by an optical pickup to read data signals from a track on the disk, it is necessary for the light beam irradiated by the optical pickup on the optical disk to be converged on the signal recording surface of the disk (focusing direction) and to be made to follow the target track (tracking direction). Hence, focusing error signals and tracking error signals are generated from the detection output of the optical pickup for driving a so-called biaxial actuator adapted to shift an objective lens directing the light beam onto the signal recording surface of the disk by way of performing focusing servo and tracking servo control operations.
As the method for detecting tracking error in the optical disk recording and/or reproducing apparatus, there are so far known such methods as a three-beam method, a push-pull method or a wobbling method.
FIG. 1 shows the basic construction of a tracking error detection apparatus operating under a three-beam system.
This tracking error detection apparatus includes, as a photo-detector unit 10 of an optical pickup detecting the return light beam from the optical disk, a main light beam detector 1 at a position of a main light spot BS.sub.1 formed by a data readout main light beam and a pair of auxiliary light beam detectors 2, 3 at the positions of auxiliary light spots BS.sub.2, BS.sub.3 formed by two auxiliary light beams adapted for tracking error detection.
The output detection currents obtained at the auxiliary light beam detectors 2, 3 as the output detection signals S.sub.2, S.sub.3 by the two auxiliary light beams by the photo-detector 10 are converted into voltage signals by means of current-to-voltage converters 11, 12, constituted by operational amplifiers 4, 5, respectively, so that the voltages are supplied to a signal subtraction circuit 13, constituted by an operational amplifier 6. The signal subtraction circuit 13 outputs a difference (S.sub.2 -S.sub.3) of the output detection signals S.sub.2, S.sub.3 of the auxiliary light beams at a signal output terminal 7 as a tracking error signal S.sub.TE.
When the light spot is moved to traverse a track on the optical disk, the tracking error signal S.sub.TE obtained at the signal output terminal 7, proves to be the traverse signal as shown at A in FIG. 2. On closing a tracking servo loop of the optical disk recording and/or reproducing apparatus, the tracking servo loop is formed so that the signal S.sub.TE is drawn towards a zero level point P.sub.z.
In practice, however, an offset DC.sub.OFF is produced in the traverse signal, obtained as the tracking error signal S.sub.TE produced at the output terminal 7, as shown at B in FIG. 2, as a result of the deviation in the split light volume ratio between the (+) first order diffracted light and the (-) first order diffracted light, produced by a diffraction grating of an optical system in the optical pickup, or the difference in the photo-electric conversion sensitivities of the auxiliary detectors 2, 3. The offset DC.sub.OFF of the traverse signal proves to be an offset of the tracking servo loop, such that the tracking by the main beam occurs off the target track on the optical disk.
With the conventional tracking error detection apparatus, for eliminating the above offset DC.sub.OFF, one of feedback resistors 8, 9, such as a resistor 9, of the operational amplifiers 4, 5 making up the current-to-voltage converters 11, 12, adapted for converting the output detection currents obtained at the auxiliary light beam detectors 2, 3 as the output detection signals S.sub.1, S.sub.2 of the two auxiliary light beams, into the corresponding voltages, is constituted as a variable resistor, so as to render the conversion constants of the current-to-voltage converting circuits 12 adjustable, by way of performing a balancing adjustment.
However, the conventional tracking error detection apparatus in which one of the feedback resistors 8, 9 of the operational amplifiers 4, 5 making up the current-to-voltage converters 11, 12 is formed as a variable resistor to eliminate the offset DC.sub.OFF by way of a balancing adjustment, is beset with an inconvenience that considerable time and labor are necessitated in the operation of the balancing adjustment.
The wobbling system for detecting the envelope of the RF level to dispense with the balancing adjustment is also unsatisfactory for practical application because of the highly complex system organization.